The present invention relates to reformers, and particularly to reforming apparatus that reforms fuel into fuel species suitable for use by electrochemical converters. In particular it relates to a plate type reformer suitable for either steam reforming or partial oxidation reforming.
The use of conventional hydrocarbon fuels as a fuel reactant for fuel cells is well known in the art. The hydrocarbon fuels are typically pre-processed and reformed into simpler reactants prior to introduction to the electrochemical converter. Conventionally, the fuel is pre-processed by passing the hydrocarbon fuel first through a desulfurization unit, then through a reformer, and a shift reactor (for H.sub.2 fueled fuel cell only) to produce a suitable fuel stock.
Conventional steam reformers currently in wide commercial use comprise a reformer section consisting of a catalyst material which promotes the reforming reaction and a burner to supply heat for the endothermic reforming reaction. A steam source is typically connected to the reformer section to provide steam. The burner typically operates at temperatures well above that required by the reforming reaction and well above the operating temperatures of conventional fuel cells, e.g., solid oxide fuel cells. Because of this, the burner must be operated as a separate unit independent of the fuel cell and as such adds considerable bulk, weight, cost and complexity to the overall power system. Furthermore, the burner is not uniquely adaptable to utilize the waste heat generally available from the fuel cell. Moreover, the consumption of extra fuel by the burner limits the efficiency of the power system.
A typical tubular reformer contains multiple tubes, which are normally made of refractory metal alloys. Each tube contains a packed granular or pelletized material having a suitable reforming catalyst as a surface coating. The tube diameter typically varies from between 9 cm and 16 cm, and the heated length of the tube is normally between 6 and 12 meters. A combustion zone is provided external to the tubes, and is typically formed in the burner. The tube surface temperature is maintained by the burner in the range of 900.degree. C. to ensure that the hydrocarbon fuel flowing inside the tube is properly catalyzed with steam at a temperature between 500.degree. C. and 700.degree. C. This traditional tube reformer relies upon conduction and convection heat transfer within the tube to distribute heat for reforming.
Plate-type reformers are known in the art, an example of which is shown and described in U.S. Pat. No. 5,015,444 of Koga et al. The reformer described therein has alternating flat gap spaces for fuel/steam mixture flow and fuel/air mixture flow. The combustion of the fuel/air stream within the spaces provides the heat for reforming of the fuel/steam mixture stream. A drawback of this design is that the reformer relies upon heat transfer between the adjacent flat gap spaces to promote the fuel reforming process.
U.S. Pat. No. 5,470,670 of Yasumoto et al. describes an integrated fuel cell/reformer structure, which has alternating layers of fuel cell and reformer plates. The heat transfer from the exothermic fuel cell to the endothermic reformer occurs through the thickness of the separating plates. A drawback of this design is that it is difficult to attain, if at all, temperature uniformities in this fuel cell/reformer structure, and which is essential in compact and efficient chemical or electrochemical apparatus designs. This fuel cell/reformer structure also requires complex and cumbersome reactant manifolding to interconnect the reactant flows between the alternating fuel cell layers and the reformer layers.
Electrochemical converters, such as fuel cells, have been known as systems for converting chemical energy derived from fuel stocks directly into electrical energy through electrochemical reaction. One type of fuel cell typically employed in fuel cell power generation systems is a solid oxide fuel cell. The solid oxide fuel cell generates electricity and releases waste heat at a temperature of about 1000.degree. C.
A typical fuel cell consists mainly of a series of electrolyte units, onto which fuel and oxidizer electrodes are attached, and a similar series of interconnectors disposed between the electrolyte units to provide serial electrical connections. Electricity is generated between the electrodes across the electrolyte by an electrochemical reaction that is triggered when a fuel, e.g., hydrogen, is introduced at the fuel electrode and an oxidant, e.g., oxygen, is introduced at the oxidizer electrode.
Typically, the electrolyte is an ionic conductor having low ionic resistance thereby allowing the transport of an ionic species from one electrode-electrolyte interface to the opposite electrode-electrolyte interface under the operating conditions of the converter. The electrical current can be tapped for external load from the interconnector plates.
The conventional solid oxide fuel cell also includes, in addition to the features listed above, an electrolyte having a porous fuel and oxidizer electrode material applied on opposing sides of the electrolyte. The electrolyte is typically an oxygen ion conducting material, such as stabilized zirconia. The oxidizer electrode, which is typically maintained in an oxidizing atmosphere, is usually an perovskite of high electrical conductivity, such as strontium doped lanthanum manganite (LaMnO3(Sr). The fuel electrode is typically maintained in a fuel rich or reducing atmosphere and is usually a cermet such as zirconia-nickel (ZrO2/Ni). The interconnector plate of the solid oxide fuel cell typically is made of an electronically conducting material which is stable in both an oxidizing and reducing atmosphere.
There still exists a need in the art for apparatus that utilizes the waste heat generated by the fuel cell for reforming use. In particular, there exists a need for employing reformer design in close association with the electrochemical converters.
The invention will next be described in connection with certain preferred embodiments. However, it should be clear that various changes and modifications can be made by those skilled in the art without departing from the spirit and scope of the invention.